Method and apparatus for transmitting data frame in wlan system

ABSTRACT

A method and device for transmitting a plurality of data units in a wireless local area network (WLAN) are discussed. The method includes obtaining by a station (STA) a transmission opportunity (TXOP), wherein the STA has a right to initiate frame exchange sequences during the TXOP; transmitting during the TXOP, a first data unit of the plurality of data units, wherein the first data unit comprises a signal field including information related to a bandwidth of the first data unit; selecting a bandwidth of a second data unit of the plurality of data units to be narrower than the bandwidth of the first data unit; and transmitting during the TXOP, the second data unit, wherein the second data unit comprises a signal field including information related to the bandwidth of the second data unit, wherein the first data unit is transmitted immediately before the second data unit.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of U.S. patent application Ser. No.16/415,431 filed on May 17, 2019, which is a Continuation of U.S. patentapplication Ser. No. 15/727,066 filed on Oct. 6, 2017 (now U.S. Pat. No.10,334,595 issued on Jun. 25, 2019), which is a Continuation of U.S.patent application Ser. No. 15/496,402 filed on Apr. 25, 2017 (now U.S.Pat. No. 9,807,765 issued on Oct. 31, 2017), which is a Continuation ofU.S. patent application Ser. No. 15/010,348 filed on Jan. 29, 2016 (nowU.S. Pat. No. 9,661,629 issued on May 23, 2017), which is a Continuationof U.S. patent application Ser. No. 14/559,688 filed on Dec. 3, 2014(now U.S. Pat. No. 9,271,191 issued on Feb. 23, 2016), which is aContinuation of U.S. patent application Ser. No. 14/186,572 filed onFeb. 21, 2014 (now U.S. Pat. No. 8,923,266 issued on Dec. 30, 2014),which is a Continuation of U.S. patent application Ser. No. 13/505,409filed on May 1, 2012 (now U.S. Pat. No. 8,699,442 issued on Apr. 15,2014), which is the National Phase of PCT/KR2011/004715 filed on Jun.28, 2011, which claims the benefit under 35 U.S.C. § 119(e) to U.S.Provisional Application No. 61/359,796 filed on Jun. 29, 2010, all ofthese applications are hereby expressly incorporated by reference intothe present application.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a Wireless Local Area Network (WLAN)system, and more particularly, to a method of a station (STA)transmitting a data frame in a WLAN system.

Discussion of the Related Art

Among the wireless communication technologies, a wireless local areanetwork (WLAN) is a technology whereby Internet access is possible in awireless fashion in homes or businesses or in a region providing aspecific service by using a portable terminal such as a personal digitalassistant (PDA), a laptop computer, a portable multimedia player (PMP),etc.

In order to overcome a limit to the communication speed that has beenconsidered as weakness in the WLAN technique, an IEEE 802.11n standardhas recently been standardized as a technology standard. The object ofthe IEEE 802.11n standard is to increase the speed and reliability of anetwork and to expand the coverage of a wireless network. Moreparticularly, in order to support a High Throughput (HT) having a dataprocessing speed of 540 Mbps or higher, minimize a transmission error,and optimize the data rate, the IEEE 802.11n standard is based onMultiple Inputs and Multiple Outputs (MIMO) technology in which multipleantennas are used on both sides of each of a transmitter and a receiver.

As the propagation of the WLAN is being activated and applicationsemploying the WLAN are being diversified, in an STA, a necessary for anew WLAN system for supporting a higher throughput than the dataprocessing speed supported by the IEEE 802.11n standard is on the rise.The next-generation WLAN system supporting a Very High Throughput (VHT)is the next version of the IEEE 802.11n WLAN system and is one of IEEE802.11 WLAN systems which have recently been proposed in order tosupport a data processing speed of 1 Gbps or higher in an MAC ServiceAccess Point (SAP).

The next-generation WLAN system supports the transmission of aMulti-User Multiple Input Multiple Output (MU-MIMO) scheme in which aplurality of non-AP STAs accesses a radio channel at the same time inorder to efficiently use the radio channel. According to the MU-MIMOtransmission scheme, an AP can transmit a frame to one or moreMIMO-paired non-AP STAs at the same time.

The AP and the plurality of MIMO-paired non-AP STAs may have differentcapabilities. A bandwidth, a Modulation Coding Scheme (MCS), and ForwardError Correction (FEC), that may be supported, may differ according to akind of the non-AP STA, purposes, a channel environment, etc. If achannel bandwidth to be used to transmit STAs having differentcapabilities can be freely controlled within a TXOP (transmissionopportunity) period, there may be generated interference with an AP oran STA or both which transmit and receive frames in a frequency band.Accordingly, reliability may become problematic when the frames aretransmitted and received. Accordingly, there is a need for a methodcapable of transmitting a data frame to STAs with different capabilitieswhile not generating interference with another AP or another STA or bothwithin the TXOP period.

SUMMARY OF THE INVENTION

The present invention provides a method in which a wireless local areanetwork (WLAN) system can transmit frames to a plurality of STA by usingthe multi user multiple input multiple output (MU-MIMO) transmissionscheme.

In an aspect, a method of transmitting a data frame in a wireless localarea network is provided. The method includes the step of: obtaining atransmission opportunity (TXOP) indicating a time interval during whicha transmitter has a right to transmit at least one data frame and anavailable bandwidth for the TXOP and sequentially transmitting aplurality of data frames to at least one receiver during the TXOP,wherein a bandwidth of a subsequent data frame of the plurality of dataframes is same to or narrower than a bandwidth of a preceding data frameof the plurality of data frames which is last previously transmittedbefore the subsequent data frame.

The step of obtaining the TXOP may include exchanging a request to send(RTS) frame and a clear to send frame (CTS).

The CTS frame may include information parameter indicating the availablebandwidth for the TXOP.

The available bandwidth for the TXOP may be same or narrower than achannel bandwidth of the RTS frame.

Each of the plurality of data frames may include a control part and adata part and the control part indicates the bandwidth of thecorresponding data frame.

Each of the plurality of data frames may be a physical layer convergenceprocedure (PLCP) protocol data unit (PPDU).

At least one of the plurality of data frames may be transmitted by usingmulti user-multiple input multiple output (MU-MIMO) transmission.

In another aspect, a wireless apparatus is provided. The apparatusincludes a transceiver configured to transmit and receive a radio signaland a processor operationally coupled to the transceiver. The processoris configured for the step of: obtaining a transmission opportunity(TXOP) indicating a time interval during which a transmitter has a rightto transmit at least one data frame and an available bandwidth for theTXOP and sequentially transmitting a plurality of data frames to atleast one receiver during the TXOP, wherein a bandwidth of a subsequentdata frame of the plurality of data frames is same to or narrower than abandwidth of a preceding data frame of the plurality of data frameswhich is last previously transmitted before the subsequent data frame.

An access point (AP) transmits a data frame using a multiple bandwidthtransmission scheme within a TXOP period. Accordingly, the entirethroughput of a WLAN system can be improved because data can betransmitted to STAs having different channel bandwidth capabilities byefficiently using the channel bandwidths within the WLAN system.

When a channel bandwidth for transmitting a data frame is selectedwithin a TXOP period, a bandwidth narrower than that used to transmit aprevious data frame is selected. Accordingly, interference between otherSTAs can be prevented when frames are transmitted and received.

A TXOP period is allocated until a data frame matched with a specificchannel bandwidth is transmitted and the data is transmitted within theTXOP period. A non-target transmission STA can access the remainingsubchannels of an idle state and can transmit and receive an additionalframe. Accordingly, the throughput can be improved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing the configuration of a WLAN system to whichembodiments of the present invention may be applied;

FIG. 2 is a block diagram showing an example of a format of a PPDUaccording to an embodiment of the present invention;

FIG. 3 is a diagram showing an example of a method of transmitting PPDUsaccording to an embodiment of the present invention;

FIG. 4 is a diagram showing another example of a method of transmittingPPDUs according to an embodiment of the present invention;

FIG. 5 is a diagram showing yet another example of a method oftransmitting PPDUs according to an embodiment of the present invention;

FIG. 6 is a diagram showing a CCA measure example which may be appliedto an embodiment of the present invention;

FIG. 7 is a flowchart illustrating a method of transmitting PPDUsaccording to an embodiment of the present invention;

FIG. 8 is a flowchart illustrating a method of transmitting PPDUsaccording to another embodiment of the present invention;

FIG. 9 shows an example of the allocation of pilot sequences accordingto channel bandwidths;

FIGS. 10 and 11 are diagrams illustrating examples in which channelsapplicable to embodiments of the present invention are used; and

FIG. 12 is a block diagram showing a wireless apparatus in which themethods of transmitting PPDUs according to the embodiments of thepresent invention may be implemented.

DETAILED DESCRIPTION OF THE EMBODIMENTS

FIG. 1 is a diagram showing the configuration of a WLAN system to whichembodiments of the present invention may be applied.

Referring to FIG. 1, WLAN system includes one or more Basic Service Set(BSSs). The BSS is a set of stations (STAs) which can communicate witheach other through successful synchronization. The BSS is not a conceptindicating a specific area.

An infrastructure BSS includes one or more non-AP STAs STA1, STA2, STA3,STA4, and STA5, an AP (Access Point) providing distribution service, anda Distribution System (DS) connecting a number of APs. In theinfrastructure BSS, an AP manages the non-AP STAs of a BSS.

On the other hand, an Independent BSS (IBSS) is operated in an Ad-Hocmode. The IBSS does not have a centralized management entity forperforming a management function because it does not include an AP. Thatis, in the IBSS, non-AP STAs are managed in a distributed manner. In theIBSS, all STAs may be composed of mobile STAs. All the STAs form aself-contained network because they are not allowed to access the DS.

An STA is a certain functional medium, including Medium Access Control(MAC) and wireless-medium physical layer interface satisfying theInstitute of Electrical and Electronics Engineers (IEEE) 802.11standard. Hereinafter, the STA refers to both an AP and a non-AP STA.

A non-AP STA is an STA which is not an AP. The non-AP STA may also bereferred to another terminology, such as a mobile terminal, a wirelessdevice, a Wireless Transmit/Receive Unit (WTRU), User Equipment (UE), aMobile Station (MS), a mobile subscriber unit, or a simply user. It ishereinafter assumed that the non-AP STA is an STA, for convenience ofdescription.

An AP is a function medium, providing access to a DS via a radio medium,for an STA associated therewith. In an infrastructure BSS including anAP, communication between STAs is in principle performed via the AP. Ifa direct link is set up between the STAs, the STAs can directlycommunicate with each other. An AP may also be referred to anotherterminology, such as a central controller, a Base Station (BS), anode-B, a Base Transceiver System (BTS), or a site controller.

A plurality of infrastructure BSSs including the BSS shown in FIG. 1 maybe interconnected through a Distribution system (DS). The plurality ofBSSs interconnected through the DS is called an Extended Service Set(ESS). An AP and/or an STA included in the ESS can communicate with eachother. In the same ESS, an STA can move from one BSS to another BSSwhile performing seamless communication.

In a WLAN system according to the IEEE 802.11 standard, a basic accessmechanism for Medium Access Control (MAC) is a Carrier Sense MultipleAccess with Collision Avoidance (CSMA/CA) mechanism. The CSMA/CAmechanism is also called the Distributed Coordination Function (DCF) ofIEEE 802.11 MAC. This mechanism basically adopts a “listen before talk”access mechanism. According to this type of an access mechanism, an APand/or an STA senses a radio channel or a medium before startingtransmission. If, as a result of the sense, the medium is determined tobe in an idle state, the AP and/or the STA starts sending a framethrough the medium. If, as a result of the sense, the medium isdetermined to be in an occupied state, the AP and/or the STA does notstart transmission and sets delay time for accessing the medium andwaits.

The CSMA/CA mechanism includes virtual carrier sensing in addition tophysical carrier sensing in which an AP and/or an STA directly senses amedium. Virtual carrier sensing is for supplement a problem that may begenerated when accessing a medium, such as a hidden node problem. Forthe virtual carrier sensing, the MAC layer of a WLAN system employs aNetwork Allocation Vector (NAV). The NAV is a value in which an APand/or an STA now using a medium or having rights to use the mediuminstructs another AP and/or another STA to use the time remaining untilthe medium becomes available. Accordingly, the value set as the NAVcorresponds to the period during which the use of the medium isscheduled by an AP or an STA or both which transmit a relevant frame.

An IEEE 802.11 MAC protocol, together with a DCF, provides a HybridCoordination Function (HCF) based on a Point Coordination Function (PCF)in which a reception AP or a reception STA or both periodically poll adata frame using the DCF and a polling-based synchronous access scheme.The HCF includes Enhanced Distributed Channel Access (EDCA) in which aprovider uses an access scheme for providing a data frame to a number ofusers as a contention-based scheme and HCF Controlled Channel Access(HCCA) employing a non-contention-based channel access scheme employinga polling mechanism. The HCF includes a medium access mechanism forimproving the Quality of Service (QoS) of a WLAN and can transmit QoSdata both in a Contention Period (CP) and a Contention-Free Period(CFP).

In the EDCA of the contention-based channel access scheme, frames having8 kinds of user priorities are allowed for differential pieces of mediumaccess. Each frame reaching the MAC layer from an upper layer has aspecific user priority value, and the MAC header of each QoS data frameincludes a user priority value.

In order to transmit the QoS data frame including the priorities, a QoSAP and/or a QoS STA implement 4 Access Categories (ACs). The userpriority of a frame reaching the MAC layer is allocated onecorresponding AC. Accordingly, if success is achieved in EDCAcontention, an EDCA TXOP (transmission opportunity) is obtained. TheTXOP is the time interval during which a specific STA has rights toinitiate transmission through a radio medium. The TXOP is used toallocate some time during which a specific AP or a specific STA or bothcan transmit a frame and to guarantee the transmission of the frame. Thetransmission start time and the maximum transmission time of the TXOPare determined by an AP. In case of the EDCA TXOP, an STA may beinformed of the TXOP through a beacon frame.

An EDCA parameter set (i.e., the core element of the EDCA scheme) is afield indicative of parameters for the traffic of a user priority. Forexample, the EDCA parameter set may be given as listed in Table 1. Forthe EDCA parameter set, reference can be made to Paragraph 7.3.2.29 of“IEEE 802.11n, Part 11: Wireless LAN Medium Access Control (MAC) andPhysical Layer (PHY) Specifications, Amendment 5: Enhancements forHigher Throughput” disclosed on October 2009.

TABLE 1 TXOP AC CWmin CWmax AIFSN limit AC_BK aCWmin aCWmax 7 0 AC_BEaCWmin aCWmax 3 0 AC_VI (aCWmin + 1)/2 − 1 aCWmin 2 3.008 ms AC_VO(aCWmin + 1)/4 − 1 (aCWmin + 1)/ 2 1.504 ms 2 − 1

Values, such as AIFSN[AC], CWmin[AC], and CWmax[AC] (i.e., the EDCAparameter set), may be carried on a beacon frame by an AP and may beinformed to each STA. Basically, priorities become higher as the valuesAIFSN[AC] and CWmin[AC] are decreased. Accordingly, a greater band isused in a given traffic environment because channel access delay isshortened. As described above, a specific STA determines thetransmission time based on the TXOP when starting transmission. An APcarries AIFSN[AC], CWmin[AC], and CWmax[AC] (i.e., EDCA parameters) andTXOP Limit[AC] (i.e., EDCA TXOP time) on a beacon frame and transfersthe beacon frame to each STA.

The TXOP may be acquired by transmitting a probe response frame,exchanging an RTS (request to send) frame and a CTS (clear to send)frame, and transmitting a CTS to self frame. Information related to theTXOP may be broadcasted by an AP and may be included in EDCA parameterset information elements included in the above frames.

Unlike in the existing WLAN system, in the next-generation WLAN system,a higher throughput is required. This is called a VHT (Very HighThroughput). To this end, the next-generation WLAN system is intended tosupport the transmission of an 80 MHz channel bandwidth, a contiguous160 MHz bandwidth, and a non-contiguous 160 MHz channel bandwidth orhigher. Furthermore, for a higher throughput, the next-generation WLANsystem provides an MU-MIMO (Multi User-Multiple Input Multiple Output)transmission scheme. In the next-generation WLAN system, an AP cantransmit a data frame to one or more MIMO-paired STAs at the same time.In a WLAN system, such as that shown in FIG. 1, an AP 10 can transmitdata to an STA group, including one or more STAs among the STAs 21, 22,23, 24 and 30 associated the AP 10, at the same time. Here, the datatransmitted to the STAs may be transmitted through different spatialstreams. The data frame transmitted by the AP 10 may be called a PPDU(Physical Layer Convergence Procedure (PLCP) Protocol Data Unit) whichis generated in the Physical Layer (PHY) of the WLAN system andtransmitted. In the examples of the present invention, it is assumedthat a target transmission STA group MU-MIMO-paired with the AP 10includes the STA1 21, the STA2 22, the STA3 23, and the STA4 24. In thiscase, data may not be transmitted to a specific STA of the targettransmission STA group because a spatial stream is not allocated to thespecific STA. Meanwhile, an STAa 30 may be associated with the AP 10,but it is assumed that the STAs 30 is not included in the targettransmission STA group.

FIG. 2 is a block diagram showing an example of a format of a PPDU 200according to an embodiment of the present invention.

Referring to FIG. 2, the PPDU 200 may include an L-STF field 210, anL-LTF field 220, an L-SIG field 230, a VHT-SIG A field 240, a VHT-STFfield 250, VHT-LTF fields 260, VHT-SIG B fields 270, and a data field280.

A PLCP sublayer constituting the PHY layer adds necessary information toa PHY Service Data Unit (PSDU) received from an MAC (Medium AccessControl) layer, converts the PSDU into the data field 280, generates thePPDU 200 by adding the L-STF field 210, the L-LTF field 220, the L-SIGfield 230, the VHT-SIG A field 240, the VHT-STF field 250, the VHT-LTFfields 260, and the VHT-SIG B fields 270 to the data field 280, andtransmits them to one or more STAs through a Physical Medium Dependent(PMD) sublayer constituting the PHY layer.

The L-STF field 210 is used for frame timing acquisition, Automatic GainControl (AGC) convergence, coarse frequency acquisition, and so on.

The L-LTF field 220 is used to estimate a channel for demodulating theL-SIG field 230 and the VHT-SIG A field 240.

An L-STA uses the L-SIG field 230 to receive the PPDU 200 and to obtaindata.

The VHT-SIG A field 240 is a field related to common control informationnecessary for STAs MIMO-paired with an AP. The VHT-SIG A field 240includes control information for interpreting the received PPDU 200. TheVHT-SIG A field 240 includes information about a spatial stream,bandwidth information, and ID information regarding whether each of aplurality of MIMO-paired STAs uses Space Time Block Coding (STBC) foreach of the plurality of MIMO-paired STAs, a group identifier (i.e., IDinformation about a target transmission STA group), information about aspatial stream allocated to an STA included in a target transmissiongroup STA indicated by a group identifier, and information related to ashort Guard Interval (GI) of a target transmission STA. Here, the groupidentifier may include information regarding whether an MIMOtransmission scheme now being used is an MU-MIMO transmission scheme ora single user (SU) MIMO transmission scheme.

The VHT-STF field 250 is used to improve the performance of AGCestimation in the MIMO transmission scheme.

The VHT-LTF fields 260 are used for an STA to estimate an MIMO channel.Since the next-generation WLAN system supports the MU-MIMO transmissionscheme, the VHT-LTF fields 260 may be set as many as the number ofspatial streams in which the PPDU 200 is transmitted. In addition, iffull channel sounding is supported and performed, the number of VHT LTFfields may be increased.

The VHT-SIG B field 270 includes dedicated control information which isnecessary for a plurality of MIMO-paired STAs to receive the PPDU 200and to acquire data. Accordingly, only when the common controlinformation included in the VHT-SIG A field 240 indicates that the PPDU200 now received has been transmitted according to the MU-MIMOtransmission scheme, an STA can be designed to receive the VHT-SIG Bfields 270. On the other hand, if the common control informationindicates that the PPDU 200 now received is for a single STA (includingthe SU-MIMO transmission scheme), an STA may be designed not to decodethe VHT-SIG B fields 270.

The VHT-SIG B field 270 includes information about the modulation,encoding, and rate-matching of each STA. The size of the VHT-SIG B field270 may be different according to the type (MU-MIMO or SU-MIMO) of MIMOtransmission and a channel bandwidth used to transmit a PPDU.

The data fields 280 include data intended to be transmitted to an STA.The data field 280 include a service field for resetting a PLCP ServiceData Unit (PSDU) to which a MAC Protocol Data Unit (MPDU) in the MAClayer has been transmitted and a scrambler, a tail field including a bitsequence necessary to return a convolution encoder to a zero state, andpadding bits for standardizing the length of a data field.

Meanwhile, in a WLAN system, such as that shown in FIG. 1, STAsassociated with an AP may have different channel bandwidth capabilities.Here, the simplest method in which the AP can transmit data to theplurality of STAs according to the MU-MIMO transmission scheme is toinclude information, indicating a channel bandwidth to be used fortransmission, in the VHT-SIG B field of a PPDU format, such as thatshown in FIG. 2. In this case, each STA can know the channel bandwidthof a PPDU transmitted thereto by decoding the VHT-SIG B field.

If a PPDU format, such as that shown in FIG. 2, is used, a channelbandwidth is included in the VHT-SIG A field 240 and transmitted. Inthis case, a plurality of STAs having different channel bandwidthcapabilities can know a common channel bandwidth. In a WLAN systemenvironment in which a plurality of STAs having different channelbandwidth capabilities coexists as described above, a method ofefficiently transmitting the PPDU during the TXOP period according tothe MU-MIMO transmission scheme needs to be discussed.

FIG. 3 is a diagram showing an example of a method of transmitting thePPDU according to an embodiment of the present invention.

Referring to FIG. 3, the AP and the plurality of MU-MIMO-paired STAs 21,22, 23, and 24 have the same channel bandwidth capability of 40 MHz.Since all the STAs have the same channel bandwidth capability, data canbe efficiently transmitted according to the MU-MIMO transmission schemeusing channel bandwidth information included in the VHT-SIG A field ofthe PPDU.

A TXOP is allocated to the STAs 21, 22, 23, and 24 associated with theAP 10 at step S310. The allocation of the TXOP may be performed wheninformation related to the TXOP acquired by the AP 10 is transmitted tothe STAs. The information related to the TXOP may be included in abeacon frame or a probe response frame and broadcasted. Furthermore, theTXOP may be allocated by exchanging an RTS (Request to Send) frame and aCTS (Clear to Send) frame. Here, a channel bandwidth available withinthe TXOP period may be determined according to a channel bandwidthparameter value included in the CTS frame.

When transmitting the PPDU within the TXOP period, the AP 10 sets thechannel bandwidth information of the VHT-SIG A field so that itindicates 40 MHz and transmits the PPDU to the plurality of STAsaccording to the MU-MIMO transmission scheme at step S320. The STAs 21,22, 23, and 24 can check a channel bandwidth value used to transmit dataincluded in the PPDU based on the channel bandwidth information includedin the received PPDU and thus can receive data. This method may alsoapply to a case where the channel bandwidth capability of an STA is an80 MHz channel bandwidth, a contiguous 160 MHz channel bandwidth, or anon-contiguous 160 MHz channel bandwidth. A channel bandwidth matchedwith the PPDU transmitted by the AP 10A may have any value smaller thanthe channel bandwidth capability.

Unlike in FIG. 3, the AP and the MU-MIMO-paired STAs may have differentchannel bandwidth capabilities. In the case where a PPDU having aformat, such as that shown in FIG. 2, is transmitted, the STAs may notbe informed of different channel bandwidths because channel bandwidthinformation is included in the VHT-SIG A field and transmitted.Accordingly, the AP includes the channel bandwidth information in theVHT-SIG A field, transmits the PPDU including the VHT-SIG A field, andtransmits the PPDU to each STA which can receive the PPDU. This isdescribed in detail with reference to FIG. 4.

FIG. 4 is a diagram showing another example of a method of transmittingPPDUs according to an embodiment of the present invention.

Referring to FIG. 4, the AP 10 and the plurality of MU-MIMO-paired STAs21, 22, 23, and 24 have different channel bandwidth capabilities, andthe different channel bandwidth capabilities may not have the samevalue.

TXOPs are allocated to the STAs 21, 22, 23, and 24 associated with theAP 10. The allocation of the TXOP may be performed when pieces ofinformation related to the TXOP acquired by the AP 10 are transmitted tothe STAs. The information related to TXOP is included in a beacon frameor a probe response frame and then broadcasted. Furthermore, the TXOPmay be allocated by exchanging an RTS (Request to Send) frame and a CTS(Clear to Send) frame. Here, a channel bandwidth available within theTXOP period may be determined according to a channel bandwidth parametervalue included in the CTS frame.

In Case 1, the AP 10 can transmit a PPDU having a 20 MHz channelbandwidth within a TXOP period at step S410. All the STA1 21, the STA222, the STA3 23, and the STA4 24 have a channel bandwidth capability ofa 20 MHz channel bandwidth or wider. Accordingly, the AP 10 can transmitthe PPDU to all the paired STAs. If it is not the case where there is nodata to be transmitted to a specific STA, a specific number of spatialstreams can be allocated to each of the STAs, and each of the STAs canreceive the PPDU through relevant spatial streams.

In Case 2, the AP 10 can transmit a PPDU having a 40 MHz channelbandwidth within a TXOP period at step S420. Here, the STA1 21 cannotreceive relevant data because it has a 20 MHz channel bandwidthcapability. Accordingly, the AP 10 transmits data to the remaining STAsother than the STA1 21. This method may be implemented in such a mannerthat a group ID indicates a target transmission STA group including theSTA1 to the STA4, but the number of spatial streams used to transmitdata to the STA1 121 is set to 0.

In Case 3, the AP 10 can transmit a PPDU having an 80 MHz channelbandwidth within a TXOP period at step S430. Here, the STA1 21 and theSTA2 22 and the STA3 23 cannot receive relevant data because the STA1 21has a 20 MHz channel bandwidth capability and the STA2 22 and the STA323 have a 40 MHz channel bandwidth capability. Accordingly, the AP 10transmits data to only the STA4 24. This method may be implemented insuch a manner that a group ID indicates a target transmission STA groupincluding the STA1 to the STA4, but the number of spatial streams usedto transmit data to the STA1 21, the STA2 22, and the STA3 23 is set to0.

As described above, the number of target transmission STAs to which anAP can transmit data according to the MU-MIMO transmission scheme ischanged according to the channel bandwidth capabilities of STAs includedin a target transmission STA group. To this end, another method oftransmitting data to a plurality of STAs having different channelbandwidth capabilities may be provided.

FIG. 5 is a diagram showing yet another example of a method oftransmitting PPDUs according to an embodiment of the present invention.

Referring to FIG. 5, the AP 10 and the plurality of MU-MIMO-paired STAs21, 22, 23, and 24 have respective channel bandwidth capabilities, andthe channel bandwidth capabilities may not have the same value.

TXOPs are allocated to the STAs 21, 22, 23, and 24 associated with theAP 10 at step S510. The allocation of the TXOPs may be performed whenpieces of information related to the TXOPs acquired by the AP 10 aretransmitted to the STAs. The information related to the TXOPs may beincluded in a beacon frame or a probe response frame and thenbroadcasted. Furthermore, the TXOPs may be allocated by exchanging aRequest to Send (RTS) frame and a Clear to Send (CTS) frame. Here, achannel bandwidth available within the TXOP period may be determinedaccording to a channel bandwidth parameter value included in the CTSframe. More particularly, the channel bandwidth available within theTXOP period may be determined from information parameter obtained byinterpreting the CTS frame received. The value set in the channelbandwidth parameter for the CTS frame may be same or smaller than that aset in the channel bandwidth for the RTS frame and may be transmittedthrough a channel bandwidth indicated by a parameter corresponding tothe CTS frame. If the channel bandwidth available within the TXOP periodis greater than a 20 MHz channel bandwidth, an AP and/or a STA cantransmit a PPDU several times using a bandwidth which is smaller than orequal to the channel bandwidth available within the TXOP period. In thefollowing embodiments, it is assumed that a channel bandwidth for a TXOPis 80 MHz.

The AP 10 transmits PPDUs to the plurality of MU-MIMO-paired STAs withinthe TXOP period at step S520. The VHT-SIG A field transmitted by the AP10 includes channel bandwidth information. The same channel bandwidth isallocated to all the STAs based on the VHT-SIG A field. Whentransmitting the PPDUs to the plurality of MU-MIMO-paired STAs withinthe TXOP period, the AP 10 transmits a PPDU suitable for the channelbandwidth capability of each STA. Here, the AP 10 may divide thetransmission period into periods and transmit PPDUs matched withdifferent channel bandwidths.

When the TXOP is set between the AP 10 and the STAs 21, 22, 23, and 24,data, control and management frames, etc. can be freely transmitted andreceived without a new contention during a specific period. First, theAP 10 transmits the PPDU, including channel bandwidth informationindicative of the 80 MHz channel bandwidth in the VHT-SIG A field, tothe STA4 24 at step S521. Next, the AP 10 transmits the PPDU, includingchannel bandwidth information indicative of a 40 MHz channel bandwidthin the VHT-SIG A field, to the STA2 22, the STA3 23, and the STA4 24 atstep S522. Next, the AP 10 transmits the PPDU, including channelbandwidth information indicative of a 20 MHz channel bandwidth in theVHT-SIG A field, to the STA1 21, the STA2 22, the STA3 23, and the STA424 at step S523. That is, the AP 10 can adapt a plurality of channelbandwidth to be transmitted according to channel bandwidth capabilitiesof each of the MU-MIMO-paired STAs.

After all data intended to be transmitted to the STA4 24 is transmittedto the STA4 24 through 80 MHz PPDU transmission when a channel bandwidthis adapted through multiple channel bandwidth transmission performedwithin the TXOP period, a PPDU needs not to be transmitted to the STA424. Accordingly, the 40 MHz PPDU may be transmitted to only the STA2 22and the STA3 23. Likewise, after all data intended to be transmitted tothe STA2 22 and the STA3 23 is transmitted to the STA2 22 and the STA323 through the 40 MHz PPDU transmission step, a PPDU needs not to betransmitted to the STA2 22 and STA3 23. Accordingly, the 20 MHz PPDU maybe transmitted to only the STA1 21.

What the AP 10 transmits the PPDUs to some of the plurality ofMU-MIMO-paired STAs may be specified by a group ID and informationindicative of the number of spatial streams allocated thereto. In a WLANsystem, such as that shown in FIG. 5, in the case where a group IDindicates an STA group including an STA1, STA2, STA3 and STA4, when thenumber of spatial streams allocated to a specific STA is set to 0, datais not transmitted to the specific STA. It means that a PPDU is notnormally transmitted to a specific STA, but means that the PPDU can betransmitted to the other specific STAs of the STA group indicated by thegroup ID.

In the case where an AP performs multiple channel bandwidth transmissionusing different channel bandwidths within the TXOP period as shown inFIG. 5, a channel bandwidth used to transmit a PPDU must be same ornarrower than a channel bandwidth used for a preceding PPDU transmitted.That is, there is proposed a method of adapting the bandwidth from awider channel bandwidth to a narrower channel bandwidth. When settingthe TXOP, a Network Allocation Vector (NAV) is set in the remaining STAsother than an STA MU-MIMO-paired with the AP, and thus the PPDU is nottransmitted to the remaining STAs. The STAa (i.e., an STA operating in adoze state) does not recognize the NAV. Consequently, the remainingsubchannels may be used by the STAa because the remaining subchannelsremain in an idle state during which the AP transmits the 20 MHz PPDU.In this case, since the AP does not perform a Clear Channel Assessment(CCA) measure within the TXOP period, a collision may occur if the APfinishes the 20 MHz PPDU transmission and transmits the 40 MHz PPDU orthe 80 MHz PPDU or both.

When the above channel bandwidth adaptation is applied, a PPDU havingthe greatest channel bandwidth is first transmitted as in the embodimentshown in FIG. 5. Referring back to FIG. 5, the AP 10 first transmits thePPDU having the 80 MHz channel bandwidth and then transmits the PPDUshaving the 40 MHz and the 20 MHz channel bandwidths.

Here, the AP has to check whether the channel band is in an idle statebefore transmitting the first PDPU within the TXOP period. For example,if the 80 MHz channel is determined to be idle within the TXOP period,the AP does not need to perform a CCA measure for the subsequent 80 MHz,40 MHz, and 20 MHz channel bandwidths and may transmit the PPDUs inorder of the 80 MHz, 40 MHz, and 20 MHz channel bandwidths. If a maximumchannel bandwidth supported by a WLAN system is further increased, theCCA measure may be performed according to a relevant channel bandwidth.

To this end, the AP performs the CCA measure based on a PPDU valuehaving the greatest channel bandwidth, from among PPDUs to betransmitted. In an embodiment for PPDU transmission, such as that shownin FIG. 5, the CCA measure regarding whether the 80 MHz channelbandwidth is available is performed. If the CCA measure for the greatestchannel bandwidth is not first performed, a CCA measure according to arelevant channel bandwidth has to be performed before a PPDU istransmitted to the relevant channel bandwidth. However, the CCA measureis not performed within the TXOP period as a general rule. This is anadvantage of the PPDU transmission/reception scheme accompanied by TXOPallocation. This is described with reference to FIG. 6.

FIG. 6 is a diagram showing a CCA measure example which may be appliedto an embodiment of the present invention.

Referring to FIG. 6, an AP checks whether frequency bands are idle whileperforming a CCA measure for a primary channel during a backoffinterval. At the same time, the AP performs a CCA measure for an 80 MHzchannel bandwidth during a Point InterFrame Space (PIFS) beforetransmitting a PPDU. If, as a result of the CCA measure, all thechannels of the 80 MHz channel bandwidth are idle during the PIFS, theAP can transmit a PPDU having the 80 MHz channel bandwidth.

In a multiple bandwidth transmission scheme in which an AP transmitsPPDUs several times by using different channel bandwidths during a TXOPperiod, the existing Short InterFrame Space (SIFS) and the existingReduced InterFrame Space (RIFS) may be applied to an InterFrame Space(IFS) when the PPDUs are transmitted.

FIG. 7 is a flowchart illustrating a method of transmitting PPDUsaccording to an embodiment of the present invention.

Referring to FIG. 7, TXOPs are allocated to the STAs 21, 22, 23, and 24associated with the AP 10 at step S710. The allocation of the TXOPs maybe performed when pieces of information related to the TXOPs acquired bythe AP 10 are transmitted to the STAs. The information related to TXOPmay be included in a beacon frame or a probe response frame and thenbroadcasted. The TXOPs may be allocated like in step S510 according tothe embodiment described with reference to FIG. 5.

The AP 10 checks whether an 80 MHz channel is idle during a contentionperiod at step S720 and then transmits PPDUs during a TXOP period atstep S730. The AP 10 transmits the PPDUs to the STAs 21, 22, 23, and 24.Here, the STAs 21, 22, 23, and 24 transmit respective acknowledgementframes ACK in response to the PPDUs received from the AP 10 at stepS740). The acknowledgement frame may be a concept including a blockacknowledgement frame. The procedure of the AP 10 transmitting the PPDUsto the STA group (21, 22, 23, and 24) transmitting the acknowledgementframes is described below.

The AP 10 first transmits an 80 MHz PPDU to the STA4 24 at step S731.When the PPDU is successfully received, the STA4 24 transmits theacknowledgement frame to the AP 10 at step S741. Next, the AP 10transmits a 40 MHz PPDU to the STA2 22, the STA3 23, and the STA4 24 atstep S732. Meanwhile, the STA2 22, the STA3 23, and the STA4 24 maytransmit the respective acknowledgement frames in response to the 40 MHzPPDU, but the AP 10 may transmit the remaining 20 MHz PPDU to the STA121, the STA2 22, the STA3 23, and the STA4 24 before the acknowledgementframes are received at step S733. After the 20 MHz PPDU is received, theSTA1 21 transmits the acknowledgement frame to the AP 10 in response tothe 20 MHz PPDU at step S742. The STA2 22, the STA3 23, and the STA4 24transmit respective block acknowledgements to the AP 10 in response tothe 40 MHz PPDU and the 20 MHz PPDU at steps S743, S744, and S745. Thesequence that the STAs transmits the respective acknowledgement framesis not limited to that shown in FIG. 7 and may be randomly determinedaccording to a channel access mechanism which is provided by a WLANsystem for transmitting acknowledgement frames.

FIG. 8 is a flowchart illustrating a method of transmitting PPDUsaccording to another embodiment of the present invention. An STA cannotreceive data which is transmitted through a wider bandwidth than itschannel bandwidth capability, but may disregard a bandwidth indicated bychannel bandwidth information included in a PPDU and receive data on thebasis of its channel bandwidth capability.

Referring to FIG. 8, the AP 10 transmits PPDUs to the plurality of STAsSTA1 21, STA2 22, the STA3 23, and the STA4 24 during a TXOP periodallocated thereto. Here, the VHT-SIG A field of the PPDU includesbandwidth information indicative of an 80 MHz channel bandwidth.Furthermore, data matched with a bandwidth according to a relevantchannel bandwidth capability is transmitted to each of the STAs.Accordingly, the AP transmits a 20 MHz PPDU to the STA1 21, a 40 MHzPPDU to the STA2 22 and the STA3 23, and the 80 MHz PPDU to the STA4 24.

Each of the STAs can receive data by using a channel bandwidth,indicated by channel bandwidth information included in the VHT-SIG Afield of the PPDU, and a channel bandwidth for data transmission, havinga smaller bandwidth from a maximum usable channel bandwidth according toits channel bandwidth capability.

There is a need for a rule for determining a channel bandwidth in orderfor an STA to receive a PPDU and to receive data on the basis of itschannel bandwidth capability as described above. For example, an STA maybe set to determine a smaller value of a signaled channel bandwidth anda maximum usable channel bandwidth as a channel bandwidth to be used.Here, the signaled channel bandwidth may be a value indicated by channelbandwidth information which is included in the VHT-SIG A field of a PPDUtransmitted by an AP.

The maximum usable channel bandwidth may correspond to the channelbandwidth capability value of a relevant STA and may be a value that istransmitted from the STA to the AP when the STA is associated with theAP. Furthermore, the maximum usable channel bandwidth may be determinedbased on channel bandwidth information included in a management actionframe informing the operating mode of an STA. Table 2 below shows aformat of an operating mode notification frame including channelbandwidth information.

TABLE 2 Order Information 1 Category 2 Action 3 Channel width

The category field is set to a value indicating that a relevant framecan be used in the next-generation WLAN system supporting the VHT. Theaction field is set to a value indicating that a relevant frame is anoperating mode notification frame. The channel width field includeschannel bandwidth information. Table 3 below shows a format of a channelbandwidth field.

TABLE 3 Value Meaning 0 20 MHz 1 40 MHz 2 80 MHz 3 160 MHz or 80 + 80MHz Other reserved

An STA can transmit the operating mode notification frame to another STAor another AP or both. The operating mode notification frame is used torestrict the channel bandwidth of a PPDU which is transmitted fromanother STA or another AP or both to a specific STA. For example, if anAP wants to receive a 20 MHz PPDU, the AP may broadcast the operatingmode notification frame to STAs within a BSS. If the AP broadcasts achannel width set to 0, the STAs within the BSS performs transmissionusing the 20 MHz PPDU. The same is true when the STA broadcasts therelevant frame.

When an AP transmits 20 MHz, 40 MHz, and 80 MHz PPDUs based on thechannel bandwidth capabilities of STAs, the AP needs to take theorthogonalities and positions of pilot sequences into consideration.

First, the pilot sequences respectively forming the 20 MHz, 40 MHz, and80 MHz channel bandwidths may not be orthogonal to each other. Second,the subcarrier positions of the pilot sequences may not be accuratelymatched with each other. That is, in different bandwidths, orthogonalityin a subcarrier in which a data tone and a pilot tone overlap with eachother cannot be guaranteed. In order to guarantee the orthogonalproperty of the data tone and the pilot tone, the data tone may bechanged into a null data tone. FIG. 9 shows an example of the allocationof pilot sequences according to channel bandwidths. When a data toneoverlapping with a pilot tone is allocated as a null data tone,orthogonality can be guaranteed.

Meanwhile, if it is an environment in which PPDU transmission accordingto the MU-MIMO transmission scheme can obtain a proper gain, an AP wouldhave already transmitted PPDUs to respective STA by properly performingbeamforming. That is, although channel bandwidths used to transmit datafrom the AP to the respective STAs are different, interferencetherebetween is small. In this case, as described above, although pilottones are not orthogonal to each other, the entire MU-MIMO transmissionperformance may not be greatly influenced.

FIGS. 10 and 11 are diagrams illustrating examples in which channelsapplicable to the embodiments of the present invention are used.

Assuming that a TXOP period is once set to an 80 MHz channel bandwidth,the AP transmits PPDUs to the STAs STAs1, 2, 3, and 4 through multiplebandwidth transmission. Although the AP transmits a 40 MHz PPDU or a 20MHz PPDU after transmitting the entire 80 MHz PPDU, other terminalscannot use redundant subchannels. In order for other terminals to haveopportunities to access the subchannels of an idle state, the TXOPperiod may be set to a PPDU transmission period up to a specific channelbandwidth.

Referring to FIG. 10, the TXOP period is set to a period during whichthe 80 MHz PPDU is transmitted to the STA4 24 and the 40 MHz PPDU istransmitted to the STA2 22, the STA3 23, and the STA4 24. The STAa notincluded in the target STA group to which the PPDUs transmitted by theAP 10 will be transmitted may perform a contention mechanism after the80 MHz PPDU and the 40 MHz PPDU are transmitted in the TXOP period,access a channel, and then transmit and receive a relevant PPDU. Thecontention mechanism may be differently performed according to a channelbandwidth to be used by the STAa. The STAa can transmit and receive therelevant PPDU using the bandwidth within a channel bandwidth having anidle state checked through the contention mechanism.

Referring to FIG. 11, a TXOP period is set to a period during which an80 MHz PPDU is transmitted to the STA4 24. The STAa may perform acontention mechanism after the 80 MHz PPDU is transmitted in the TXOPperiod, access a channel, and then transmit and receive a relevant PPDU.The contention mechanism may be differently performed according to achannel bandwidth to be used by the non-AP STAa. The STAa can transmitand receive the relevant PPDU using the bandwidth within a channelbandwidth having an idle state checked through the contention mechanism.

FIG. 12 is a block diagram showing a wireless apparatus in which themethods of transmitting PPDUs according to the embodiments of thepresent invention may be implemented.

Referring to FIG. 12, the wireless apparatus 1200 includes a processor1210, memory 1220, and a transceiver 1230. The transceiver 1230transmits and/or receives a radio signal and implements the physicallayer of the IEEE 802.11 standard. The processor 1210 is functionallyconnected to the transceiver 1230 and is set to implement the MAC layeror the PHY layer or both for implementing the embodiments of the presentinvention shown in FIGS. 2 to 11, in which a data frame, such as a PPDUformat, is generated, a transmission channel is selected, and the dataframe is transmitted through the transmission channel. The processor1210 and/or the transceiver 1230 may include an application-specificintegrated circuit (ASIC), a separate chipset, a logic circuit, and/or adata processing unit. When the embodiment of the present invention isimplemented in software, the aforementioned methods can be implementedwith a module (i.e., process, function, etc.) for performing theaforementioned functions. The module may be stored in the memory 1220and may be performed by the processor 1210. The memory 1220 may belocated inside or outside the processor 1210, and may be coupled to theprocessor 1210 by using various well-known means.

What is claimed is:
 1. A method of transmitting a plurality of dataunits in a wireless local area network (WLAN), the method comprising:obtaining, by a station (STA), a transmission opportunity (TXOP),wherein the STA has a right to initiate frame exchange sequences duringthe TXOP; transmitting, by the STA during the TXOP, a first data unit ofthe plurality of data units, wherein the first data unit comprises asignal field including information related to a bandwidth of the firstdata unit; selecting, by the STA, a bandwidth of a second data unit ofthe plurality of data units to be narrower than the bandwidth of thefirst data unit; transmitting, by the STA during the TXOP, the seconddata unit, wherein the second data unit comprises a signal fieldincluding information related to the bandwidth of the second data unit,wherein the first data unit is transmitted immediately before the seconddata unit; selecting, by the STA, a bandwidth of a third data unit ofthe plurality of data units to be narrower than the bandwidth of thesecond data unit; and transmitting, by the STA during the TXOP, thethird data unit, wherein the third data unit comprises a signal fieldincluding information related to the bandwidth of the third data unit,wherein the second data unit is transmitted immediately before the thirddata unit.
 2. The method of claim 1, wherein the STA performs a clearchannel assessment (CCA) for the bandwidth of the first data unit beforetransmitting the first data unit.
 3. The method of claim 1, wherein theSTA is an Access Point (AP) STA or a non-AP STA.
 4. The method of claim1, wherein the bandwidth of the first data unit is 80 MHz, the bandwidthof the second data unit is 40 MHz, and the bandwidth of the third dataunit is 20 MHz.
 5. The method of claim 1, wherein each of the first,second and third data units is a physical protocol data unit (PPDU)comprising a data frame.
 6. A station (STA) in a wireless local areanetwork (WLAN), comprising: a transceiver; and a processor operativelyconnected to the transceiver and configured to: obtain a transmissionopportunity (TXOP), wherein the STA has a right to initiate frameexchange sequences during the TXOP; transmit, during the TXOP, a firstdata unit of a plurality of data units, wherein the first data unitcomprises a signal field including information related to a bandwidth ofthe first data unit; select a bandwidth of a second data unit of theplurality of data units to be narrower than the bandwidth of the firstdata unit; transmit, during the TXOP, the second data unit, wherein thesecond data unit comprises a signal field including information relatedto the bandwidth of the second data unit, wherein the first data unit istransmitted immediately before the second data unit; select a bandwidthof a third data unit of the plurality of data units to be narrower thanthe bandwidth of the second data unit; and transmit, during the TXOP,the third data unit, wherein the third data unit comprises a signalfield including information related to the bandwidth of the third dataunit, wherein the second data unit is transmitted immediately before thethird data unit.
 7. The STA of claim 6, wherein the STA is furtherconfigured to perform a clear channel assessment (CCA) for the bandwidthof the first data unit before transmitting the first data unit.
 8. TheSTA of claim 6, wherein the STA is an Access Point (AP) STA or a non-APSTA.
 9. The STA of claim 6, wherein the bandwidth of the first data unitis 80 MHz, the bandwidth of the second data unit is 40 MHz, and thebandwidth of the third data unit is 20 MHz.
 10. The STA of claim 6,wherein each of the first, second and third data units is a physicalprotocol data unit (PPDU) comprising a data frame.
 11. A method oftransmitting a plurality of data units in a wireless local area network(WLAN), the method comprising: obtaining, by a station (STA), atransmission opportunity (TXOP), wherein the STA has a right to initiateframe exchange sequences during the TXOP; transmitting, by the STAduring the TXOP, a first data unit of the plurality of data units,wherein the first data unit comprises a signal field includinginformation related to a bandwidth of the first data unit; selecting, bythe STA, a bandwidth of a second data unit of the plurality of dataunits from first available bandwidths, wherein the first availablebandwidths include a first bandwidth which is same as the bandwidth ofthe first data unit and a second bandwidth which is narrower than thebandwidth of the first data unit; transmitting, by the STA during theTXOP, the second data unit, wherein the second data unit comprises asignal field including information related to the bandwidth of thesecond data unit, wherein the first data unit is transmitted immediatelybefore the second data unit; selecting, by the STA, a bandwidth of athird data unit of the plurality of data units from second availablebandwidths, wherein the second available bandwidths include a thirdbandwidth which is same as the bandwidth of the second data unit and afourth bandwidth which is narrower than the bandwidth of the second dataunit; and transmitting, by the STA during the TXOP, the third data unit,wherein the third data unit comprises a signal field includinginformation related to the bandwidth of the third data unit, wherein thesecond data unit is transmitted immediately before the third data unit.12. The method of claim 11, wherein the STA performs a clear channelassessment (CCA) for the bandwidth of the first data unit beforetransmitting the first data unit.
 13. The method of claim 11, whereinthe STA is an Access Point (AP) STA or a non-AP STA.
 14. The method ofclaim 11, wherein the bandwidth of the first data unit is 80 MHz, thebandwidth of the second data unit is 40 MHz, and the bandwidth of thethird data unit is 20 MHz.
 15. The method of claim 11, wherein each ofthe first, second and third data units is a physical protocol data unit(PPDU) comprising a data frame.
 16. A station (STA) in a wireless localarea network (WLAN), comprising: a transceiver; and a processoroperatively connected to the transceiver and configured to: obtain atransmission opportunity (TXOP), wherein the STA has a right to initiateframe exchange sequences during the TXOP; transmit, during the TXOP, afirst data unit of the plurality of data units, wherein the first dataunit comprises a signal field including information related to abandwidth of the first data unit; select a bandwidth of a second dataunit of the plurality of data units from first available bandwidths,wherein the first available bandwidths include a first bandwidth whichis same as the bandwidth of the first data unit and a second bandwidthwhich is narrower than the bandwidth of the first data unit; transmit,during the TXOP, the second da to unit, wherein the second data unitcomprises a signal field including information related to the bandwidthof the second data unit, wherein the first data unit is transmittedimmediately before the second data unit; select a bandwidth of a thirddata unit of the plurality of data units from second availablebandwidths, wherein the second available bandwidths include a thirdbandwidth which is same as the bandwidth of the second data unit and afourth bandwidth which is narrower than the bandwidth of the second dataunit; and transmit, during the TXOP, the third data unit, wherein thethird data unit comprises a signal field including information relatedto the bandwidth of the third data unit, wherein the second data unit istransmitted immediately before the third data unit.
 17. The STA of claim16, wherein the STA is further configured to perform a clear channelassessment (CCA) for the bandwidth of the first data unit beforetransmitting the first data unit.
 18. The STA of claim 16, wherein theSTA is an Access Point (AP) STA or a non-AP STA.
 19. The STA of claim16, wherein the bandwidth of the first data unit is 80 MHz, thebandwidth of the second data unit is 40 MHz, and the bandwidth of thethird data unit is 20 MHz.
 20. The STA of claim 16, wherein each of thefirst, second and third data units is a physical protocol data unit(PPDU) comprising a data frame.